Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

1. Setting the Scope of the Challenge 6474 1.1. The Need for Solar Energy Supply and Storage 6474 1.2. An Imperative for Discovery Research 6477 1.3. Scope of Review 6478 2. Large-Scale Centralized Energy Storage 6478 2.1. Pumped Hydroelectric Energy Storage (PHES) 6479 2.2. Compressed Air Energy Storage (CAES) 6480 3. Smaller Scale Grid and Distributed Energy Storage 6481 3.1. Flywheel Energy Storage (FES) 6481 3.2. Superconducting Magnetic Energy Storage 6482 4. Chemical Energy Storage: Electrochemical 6482 4.1. Batteries 6482 4.1.1. Lead-Acid Batteries 6483 4.1.2. Alkaline Batteries 6484 4.1.3. Lithium-Ion Batteries 6484 4.1.4. High-Temperature Sodium Batteries 6484 4.1.5. Liquid Flow Batteries 6485 4.1.6. Metal-Air Batteries 6485 4.2. Capacitors 6485 5. Chemical Energy Storage: Solar Fuels 6486 5.1. Solar Fuels in Nature 6486 5.2. Artificial Photosynthesis and General Considerations of Water Splitting 6486

Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds

The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting

Earth-abundant first-row (3d) transition metal-based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodynamic requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 millivolts) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation after more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER.

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Homogeneously dispersed, multimetal oxygen-evolving catalysts

Hydrogen from solar-driven water splitting has the potential to provide clean energy. Current progress towards artificial photosynthetic devices is reviewed, with particular focus on visible light active nanostructures. A vision for a future sustainable hydrogen fuel community based on artificial photosynthesis is outlined.

Artificial photosynthesis for solar water-splitting

A series of silicon clusters containing four atoms but with different charge states (Si4(2+), Si4, Si4(2-), and NaSi4-) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change upon addition or removal of two electrons. It is shown that Si4(2+) is square-planar, analogous to the recently discovered aromatic Al4(2-) cluster. Upon of two electrons, neutral Si4 becomes sigma-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si4(2-): either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si4(2-), a stabilizing cation (Na+) was used to produce Si4(2-) in the gas phase in the form of Na+[Si4(2-)], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+[Si4(2-)] species is highly unusual.

http://casey.brown.edu/chemistry/research/LSWang/publications/188.pdf

Multiple Aromaticity and Antiaromaticity in Silicon Clusters

A single π-bond captures 3, 4 and 5 atoms

Revealing substituent effects on the electronic structure and planarity of Ni-porphyrins

Geometry and electronic structure of five species ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2) 10- (1), ({Ru4O4(OH)2(H2- O)4}(γ-SiW10O36)2) 9- (2), ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2) 8- (3), ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2) 7- (4), and ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2) 6- (5) with different oxidation states of Ru centers were studied at the density functional and COSMO levels of theory. These species are expected to be among the possible intermediates of the recently reported 1-catalyzed water oxidation (Geletii, Y. V.; Botar, B.; Kogerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. Angew. Chem. Int. Ed. 2008, 47, 3896-3899 and Sartorel, A.; Carraro, M.; Scorrano, G.; Zorzi, R. D.; Geremia, S.; McDaniel, N. D.; Bernhard, S.; Bonchio, M. J. Am. Chem. Soc. 2008, 130, 5006-5007). It was shown that RI-BP86 correctly describes the geometry and energy of the low-lying electronic states of compound 1, whereas the widely used B3LYP approach overestimates the energy of its high-spin states. Including the solvent and/or countercation effects into calculations improves the agreement between the calculated and experimental data. It was found that the several HOMOs and LUMOs of the studied complexes are bonding and antibonding orbitals of the (Ru4O4(OH)2(H2O)4) 6+ core, and four subsequent one-electron oxidations of 1, leading to formation of 2, 3, 4, and 5, respectively, involve only {Ru4} core orbitals. In other words, catalyst instability due to ligand oxidation in the widely studied Ru-blue dimer, ((bpy)2(O)Ru V -(µ-O)-Ru V (O)(bpy)2) 4+ , is not operable for 1: the latter all-inorganic catalyst is predicted to be stable under water oxidation turnover conditions. The calculated HOMOs and LUMOs of all the studied species are very close in energy and exhibit a "quasi-continuum" or "nanoparticle-type" electronic structure similar to that of nanosized transition metal clusters. This conclusion closely correlates with the experimentally reported oxidation and reduction features of 1 and explains the unusual linear dependence of oxidation potential versus charges for these compounds. The decrease in total negative charge of the system via 1 > 2 > 3 > 4 > 5, on average, decreases the {Ru4}-{SiW10} distance. It is predicted that at higher pH compound 1 will, initially, release protons from the µ-ORu oxygen centers.

Computational studies of the geometry and electronic structure of an all-inorganic and homogeneous tetra-Ru-polyoxotungstate catalyst for water oxidation and its four subsequent one-electron oxidized forms.

Recently, carbon allotropes were shown to play a key role in energy harvesting and as hydrophobic coatings on metal alloys. We have designed octylamine-functionalized graphene oxide materials for energy harvesting and as an anti-corrosion coating for metal alloy protection in a 3.5% NaCl medium. The material has been characterized by different techniques to confirm the structure and composition of the modified graphene oxide sheet: FTIR spectroscopy, XRD, Raman spectroscopy, FESEM and TEM. The electrochemical stability and corrosion inhibition efficiency were studied by electrochemical methods. The electrochemical stability increased with an increase in the applied voltage up to 500 mV, and the corrosion inhibition efficiency was shown to be 73%. The coating stability studies showed a long stability time in the corrosion medium.

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Experimental and computational studies of graphene oxide covalently functionalized by octylamine: electrochemical stability, hydrogen evolution, and corrosion inhibition of the AZ13 Mg alloy in 3.5% NaCl

Aleksey E. Kuznetsov

Papers

Multiple Aromaticity and Antiaromaticity in Silicon Clusters

A single π-bond captures 3, 4 and 5 atoms

Revealing substituent effects on the electronic structure and planarity of Ni-porphyrins

Computational studies of the geometry and electronic structure of an all-inorganic and homogeneous tetra-Ru-polyoxotungstate catalyst for water oxidation and its four subsequent one-electron oxidized forms.

Experimental and computational studies of graphene oxide covalently functionalized by octylamine: electrochemical stability, hydrogen evolution, and corrosion inhibition of the AZ13 Mg alloy in 3.5% NaCl